Abstract : Aircraft performance can be defined as the ability of an aircraft to climb, accelerate, or maneuver in order to successfully accomplish its mission. Obviously, expected performance specifications must be an integral part of the design process of an aircraft. Given certain performance expectations by the customer, the designer must make decisions regarding wing loading, power plant selection, airfoil selection, planform configuration, and many other considerations. All of these help to tailor the design to give the aircraft the desired performance characteristics. It is also certain that actual performance characteristics will not always be the same as those predicted by the designer. Herein lies the need for performance flight testing. Performance flight testing is defined as the process of determining aircraft performance characteristics, or in a more modern sense, evaluation of the energy gaining and losing capability of the aircraft. Determination of aircraft performance is dependent upon our knowledge of fundamentals in several different scientific disciplines. In order to predict or measure an aircraft's performance, we must be able to estimate the aerodynamic forces involved. This requires knowledge of the properties and behavior of the fluid medium in which we operate, i.e., the earth's atmosphere. Therefore, we must study atmospheric science, fluid dynamics, thermodynamics, and aerodynamics. Performance prediction or measurement requires knowledge of the aircraft propulsion system. Hence, we must be familiar with the theory and operation of basic turbine and turbine variant engines, reciprocating internal-combustion engines, and propeller theory. We must also understand the basic measurements, instrumentation techniques, and equipment used to gather the data needed to determine an aircraft's performance.